The seismic exploration industry uses acoustic impulses to impart sonic vibrations into the earth to delineate subsurface structure for mineral exploration and development. These acoustic impulses may be from an explosive, implosive, swept-frequency (chirp) or random source. A recording of the acoustic reflection and refraction wavefronts that travel from the source to a receiver is used to produce a seismic field record. Variations in the travel times of reflection and refraction events in these field records indicate the position of reflection surfaces within the earth. The analysis and correlation of events in one or more field records in seismic data processing produces an acoustic image that demonstrates subsurface structure. The acoustic images are used to find valuable mineral deposits.
The swept-frequency or chirp type seismic source may utilize a relatively long pilot signal such as 2 to 15 seconds to assure sufficient energy is imparted to the earth. The swept-frequency or chirp type source method relies on signal compression to compress the signal and ensure sufficient vertical resolution to resolve the position of subsurface reflectors. Signal compression generally is called deconvolution, with many techniques well known in the art of seismic data processing. Deconvolution of sweep or chirp signals compresses the source signal into a much shorter signal representative of a subsurface reflective boundary. The accuracy and effectiveness of any deconvolution technique is directly related to how well the source signal is known or understood. Most deconvolution operators are derived from statistical estimates of the actual source waveform.
With a swept frequency type source the energy is emitted in the form of a sweep of regularly increasing (upsweep) or decreasing (downsweep) frequency in the seismic frequency range. The vibrations are controlled by a control signal, which can control the frequency and phase of the seismic signals.
When only one seismic source is used, the seismic surveys can be very time-consuming. With modern signal processing methods this period could be shortened if more than one seismic source could be used simultaneously. Multiple sources can be used if some means for distinguishing between signals emanating from the different sources can be provided. Further, the vibrational source generates harmonics which, in certain circumstances, can have an energy approaching or even exceeding the fundamental, and which can crossfeed with signals from other sources, giving misleading results when the signals are processed to separate the signals from each source. In addition, the harmonics are a source of noise and can mask weak reflection signals from deeper layers.
U.S. Pat. No. 3,885,225 to Anstey et al proposes a method to distinguish between multiple sources. Anstey et al is directed to a method and apparatus for broad-line seismic profiling, using several vibrators simultaneously emitting signals. The normal emission frequency bandwidth is divided into several parts that are allocated to individual vibrators in a sequence of separate emissions, in such a way that the several vibrators radiate mutually exclusive frequencies at any one time. The detected signals are separated on the basis of frequency to represent the individual signals from each vibrator. The frequency limitation on each individual vibrator reduces the sensitivity of the survey. Harmonic distortion in the vibrators or their coupling with the ground can inhibit signal separation from different generators, this '225 patent makes no attempt to remove or reduce this distortion.
Another method of signal separation from multiple vibratory sources using phase shifting of the signals on different sweeps is disclosed in U.S. Pat. No. 4,715,020 to Landrum. However, the problem of nonlinear distortion and crossfeed is not addressed in this patent.
A method for attenuating harmonic correlation noise caused by harmonic energy output from seismic vibrators was developed by Reitsch as disclosed in U.S. Pat. No. 4,042,910. The method includes the step of generating a plurality of sweep signals in series and with the phase of each succeeding sweep signal being shifted relative to the previous one by a predetermined phase angle that is a fraction of 2π. The generated signals are separately recorded and transformed by inverse phase shifting before being added or stacked in a conventional manner. Using this method, a series of N sweep segments are output by the vibrators (one for each record) that differ only in phase. Correlation noise up the Nth harmonic is attenuated. This method provides a method of suppressing harmonics using phase shifting, but only for a single vibratory source, and crossfeed is not addressed.
U.S. Pat. No. 4,823,326 to Ward, claims a method for separating seismic records derived from multiple, concurrently operated vibrational seismic sources, with reduced harmonic distortion.
U.S. Pat. No. 4,982,374 to Edington and Khan is a method for reducing the distortion and crossfeed from any selected order harmonic for any number of vibratory seismic sources operated concurrently, at the same time providing for separation of the signals from the different sources and for improving the signal-to-noise ratio. After determining the highest order harmonic likely to cause distortion, a number of sweeps of each source in each position is selected. This number depends upon the number of sources and the highest order harmonic to be suppressed. Initial phase angles for each sweep of each source are then selected to permit signal separation while suppressing harmonics up to and including that highest order harmonic.
U.S. Pat. No. 5,410,517 to Andersen discloses a method of cascading sweep segments to suppress unwanted harmonic energy. The method uses sweep segments having varying phase angles such that harmonic energy in the correlated wavelets is attenuated. According to the method, a first cascaded sweep sequence is generated containing N sweep segments linked end-to-end. The N sweep segments are substantially identical, except that the initial phase angle of each sweep segment within the sweep sequence is progressively rotated by a constant phase increment of about 360/N degrees. A second cascaded sweep sequence is generated consisting of: (i) N consecutive sweep segments linked end-to-end which correspond to said first cascaded sweep sequence, and (ii) an additional sweep segment linked to the N consecutive sweep segments which is positioned and phased so as to substantially suppress harmonic ghosts during correlation. One of these cascaded sweep sequences is used for the vibrator sweep sequence and the other is used for the correlation reference sequence.